1. Field of the Invention
The present invention generally relates to a planar antenna, and more particularly, to a planar antenna with an isotropic radiation pattern.
2. Description of Related Art
The isotropic radiation pattern can prevent deterioration of communication quality caused by nulls. Thus, antennas with the isotropic radiation pattern are very adaptable to communication products, especially handheld products (for example, cell phones, notebook computers, portable mobile communication devices, Bluetooth devices, or WiFi devices), for receiving or transmitting wireless signals from or to all directions. FIG. 1 illustrates the structure of a conventional antenna with the isotropic radiation pattern. Referring to FIG. 1, the antenna 100 includes a substrate 110, a dipole antenna 120, a spiral radiating body 130, and another spiral radiating body 140. The dipole antenna 120 is disposed on a first surface 111 of the substrate 110, and the spiral radiating bodies 130 and 140 are respectively disposed on a second surface of the substrate 110. For the convenience of description, the corresponding positions of the spiral radiating bodies 130 and 140 on the first surface 111 of the substrate 110 are perspectively denoted with doted lines.
Referring to FIG. 1, the spiral radiating bodies 130 and 140 are symmetrical to each other and electrically connected to two radiating bodies 121 and 122 in the dipole antenna 120 respectively through a via 151 and a via 152. Based on the Ampere's right-hand rule, the magnetic fields produced by the spiral radiating bodies 130 and 140 run through the first surface 111 (i.e., the magnetic field directions M12 and M13) with the current direction D11 and form a magnetic dipole. Besides, the direction of the magnetic dipoles produced by the spiral radiating bodies 130 and 140 is perpendicular to that of the electric dipole produced by the dipole antenna 120. Thus, the antenna 100 can generate two orthogonal radiation patterns through the spiral radiating bodies 130 and 140 and the dipole antenna 120 and accordingly produce the isotropic radiation pattern due to the mutual compensation of the two orthogonal radiation patterns.
To be specific, the spiral radiating body 130 is composed of three microstrip lines 131˜133 that are connected with each other in series. The microstrip line 132 presents a narrow arc shape (for example, a narrow transmission line) therefore can relatively block high-frequency signals. The impedance X of the microstrip line 132 satisfies X=ωL=(2πf)L, therefore the impedance X is in direct proportion to the frequency f and the inductance value L, which means the higher the frequency f or inductance L is, the greater the impedance X will be and accordingly the harder for high-frequency signals to pass through, wherein the length of the micro strip line 132 should be shorter than λg/4 wherein λg is a guided wavelength. In other words, the microstrip line 132 is like an inductive filter, wherein the low-frequency signals from the microstrip line 131 can pass through the microstrip line 132 and reach the microstrip line 133, but the high-frequency signals from the microstrip line 131 cannot pass through the microstrip line 132. Accordingly, a high-frequency path is formed by the radiating body 121 and the microstrip line 131 that are connected with each other in series, and a low-frequency path is formed by the radiating body 121 and the microstrip lines 131˜133 that are connected with each other in series. Thereby, the antenna 100 with the isotropic radiation pattern can receive and transmit dual band signals.
Besides, the narrower width of the microstrip line 132 is, the higher inductance value L and hence the better blocking ability of the high-frequency will be. However, it should be noted that because the minimum width of the microstrip line 132 is limited by the printing technique on the substrate 110, the capability of blocking high-frequency signals is thus also limited by the printing technique on the substrate 110. In addition, if the microstrip line 132 is disposed at a fixed position, the antenna 100 with the isotropic radiation pattern can only be applied to limited types of channels (i.e., channel selection cannot be carried out) within the high and low frequency paths. Moreover, due to the narrow width of the microstrip line 132 with large inductance value L to do better blockage of high-frequency signals, the energy loss will hence increase. In other words, the radiation efficiency of the isotropic antenna 100 is reduced.